A conventional ball valve is known to comprise in interior space of its body a structure with a valve disc (ball) opening and closing a flow channel by rotating about a stem shaft and a ball seat arranged so as to surround a periphery of the valve disc. In a ball valve having such structure, it is necessary to press the ball seat to the ball side in order to prevent leakage.
In Japan Patent Tokukai 2011-174598 (patent document 1), as shown in FIG. 4, discloses a ball valve (1′) having a structure which sandwiches a ball seat (12′) by a ball seat pressing part (14′) and a ball seat receiving surface (13′) inserted into a cylindrical part of a valve body (2′), and accommodates a ball (3′) in the ball seat.
The ball valve (1′) in the patent document 1 comprises a fluororesin packing (15′) arranged above the ball seat pressing part (14′) to seal a gap between a stem (5′) and the cylindrical part of the body, and a packing gland (16′) arranged above the fluororesin packing, with the ball seat pressing part (14′) pressed downward via these by a packing gland screw (17′) which was screwed to fit in the inner periphery of the cylindrical part of the body. Therefore, the leakage can be prevented.
If the fluororesin packing (15′) is not pressed enough, however, it may absorb displacement due to its elasticity without sufficiently conveying the force to the ball seat pressing part, thereby leading to a leakage. There was a problem that if the packing gland screw (17′) is tightened in order to prevent the leakage, the handle would become heavy and opening and closing of the valve by hand would be difficult.